Galectin-9 is a mammalian lectin with a molecular weight of 40 kD that has two conserved carbohydrate recognition domains (CRDs) and forms homodimers. It recognizes N-acetyllactosamine (Galbeta1-4GlcNAc) and T-antigen (Galbeta1-3GalNAc); Tim-3 has been reported as its ligand. Galectin-9 is expressed by lymphocytes, dendritic cells, granulocytes, eosinophils, astrocytes, endothelial cells, fibroblasts and thymus epithelial cells. It may be retained intracellularly or be transported to the cell surface where it is cleaved, thereby generating a soluble form. Galectin-9 is involved in events such as cell aggregation, cell adhesion, chemotaxis and apoptosis. Importantly for the regulation of the immune response, Galectin-9 induces regulatory T cells and suppresses Th1 and Th17 responses.